Field of the Invention
The invention relates to a process for the selective catalytic reduction of nitrogen oxides in an oxygen-containing gaseous medium. The process uses a substance which can be converted into a reducing agent. The substance is first converted into the reducing agent by substantially thermal means in a separate preparation reactor and then the reducing agent is added to the gaseous medium upstream of a reduction catalytic converter in order to react with the nitrogen oxides. The gaseous medium is, for example, the exhaust gas from a combustion system or an oxygen-containing process gas from a commercial or industrial plant, e.g. from the chemical industry. In principle, it is possible to treat any oxygen-containing gas which is to be deNOxed. Furthermore, the invention relates to a device for carrying out the above process.
The process of selective catalytic reduction (SCR) for eliminating nitrogen oxides from an oxygen-containing gaseous medium has by now become established as a process which can be successfully managed on an industrial scale. The process is used in particular for deNOxing exhaust gases from combustion systems, including diesel engines. In the process, the nitrogen oxides contained in the exhaust gas are reacted to form molecular nitrogen and water in the presence of oxygen on a selectively operating reduction catalytic converter by means of a suitable reducing agent. A catalyst based on titanium oxide which also contains additions of molybdenum trioxide, tungsten trioxide and/or vanadium pentoxide, has proven suitable for the reduction catalytic converter. Ammonia (NH3) has proven to be the most effective and successful reducing agent, compared to other reducing agents such as hydrocarbons or cyanuric acid.
However, since ammonia is a strongly smelling and, at high concentrations, also a toxic compound, there are regulations on safe storage and handling which are specific to individual countries. In particular, considerable safety precautions, which entail costs, have to be taken if ammonia is to be carried along in a suitably protected tank to be used as a reducing agent for cleaning the exhaust gases from diesel-powered passenger automobiles, commercial vehicles, or other vehicles.
One possible solutions to the problem is described in a Siemens brochure “SINOX, Stickoxidminderung für stationäre Dieselmotoren” [SINOX, Abatement of Nitrogen Oxides for Stationary Diesel Engines], Order No. A 96001-U91-A232, 1997, in which it is proposed to use a substance which can be converted into ammonia, namely urea, instead of ammonia. This is because compared to ammonia urea is altogether harmless when being transported and stored. To reduce the nitrogen oxides, an aqueous urea solution is thereby sprayed directly into the exhaust gas line upstream of the reduction catalytic converter. On account of the relatively high exhaust gas temperatures, the urea is broken down into ammonia by pyrolysis and by hydrolysis. The resulting ammonia is then reacted with the nitrogen oxides as the actual reducing agent at the reduction catalytic converter in the manner described above.
However, with a process of that nature a series of technical questions remain unanswered. For example, breaking down urea in the gas stream involves the problem of the formation of undesirable byproducts, which in some cases form solid, insoluble deposits on components downstream of the reaction location. Undesirable byproducts of this nature include cyanuric acid, from which insoluble melamines are formed, or ammonium sulfates, which are formed if sulfur oxides are present. In addition, the urea may also crystallize out before or during the injection into the gas stream and may lead to the parts which carry urea becoming blocked.
To solve these problems, it has become known for the urea to be particularly finely atomized during its introduction into the gas stream, to use a suitable catalyst in order to assist hydrolysis, or to spray the urea onto a vaporizer arranged in the gas stream in order to achieve rapid decomposition. Solutions of this type are known, for example, from European patent EP 0 487 886 B1 for an exhaust gas stream.
Particularly in a large-scale plant, such as a fossil-fired power plant, in which, for treating the exhaust gas, the reducing agent has to be introduced into the exhaust gas by means of an injection grate which comprises a multiplicity of nozzles, in order to achieve a uniform distribution in the exhaust gas duct, however, when urea is used it is still necessary, as before, for each individual nozzle to be controlled separately, in order, in the event of one nozzle becoming blocked, to be able to balance the amount of reducing agent required by supplying more to the other nozzles. This is because even the measures listed cannot rule out the possibility of a nozzle becoming blocked.
As an alternative to the urea being broken down in the exhaust gas stream, it is also described, in the European patent EP 0 487 886 B1, for the urea to be broken down outside the exhaust gas stream. A urea solution is thereby applied to a heated vaporizer in a separate preparation reactor, by means of a spray device, for pyrolysis, and the gas mixture formed is passed over a downstream hydrolysis catalytic converter. Only then is the gas mixture of ammonia and residues which is formed introduced into the exhaust gas stream. The amount of ammonia metered in is regulated by regulation of the amount of gas mixture.
In this way, undesirable byproducts are not formed directly inside the exhaust gas stream from the combustion system, but rather outside this stream, where fewer adverse effects are to be expected.
With a procedure of this type, however, the vaporizer must be heated to the temperature required for hydrolysis with additional energy. This is because, unlike a vaporizer situated in the exhaust gas stream, the separate vaporizer is not preheated by the hot exhaust gas flowing past it. To achieve economic operation, a rather small area is selected for the vaporizer, so that it is possible to achieve the hottest possible surface of the vaporizer with the lowest possible energy consumption.
In a large-scale industrial plant in which the exhaust gas duct has a large cross section and/or in which there is a high volumetric flow rate of exhaust gas, such as a fossil-fired power plant or a gas turbine or a large-scale plant used in the chemical industry in which a large amount of process gas which is to be deNOxed is produced per unit time, however, the surface area of the vaporizer has to be selected to be sufficiently large to ensure that the urea is broken down into ammonia as completely as possible. However, to heat a large-area vaporizer of this nature to the temperature required for conversion into the reducing agent, a high additional input of energy is required. This makes the process according to the prior art uneconomical for large-scale plants, for which reason it is not used, for example, for gas turbines and fossil-fired power plants with large exhaust gas duct cross sections or large-scale plants used in the chemical industry with a high quantity of process gas which has to be deNOxed.